The endocannabinoid (eCB) system participates in behavioral adaptations that accompany chronic exposure to opiates; the cellular bases for these changes are not known. One form of eCB signaling involves the postsynaptic production of eCBs that diffuse in a retrograde manner to act on presynaptic CB1 receptors to inhibit neurotransmitter release. In the prior funding period a cell culture model that displays robust eCBmediated depolarization-induced suppression of excitatory synaptic transmission (DSE) was developed. Preliminary data show that this model is well suited for examining adaptive changes in retrograde eCB signaling between hippocampal neurons. Three specific aims will examine the effects of prolonged exposure to opiates on the eCB system. 1) Opiates exert marked effects on synaptic transmission between hippocampal neurons. The hypothesis that prolonged exposure to opiates will up-regulate eCB signaling will be tested. 2) Chronic exposure to opiate agonists increases the expression of brain derived neurotrophic factor (BDNF) and increases excitatory synaptic transmission. The hypothesis that BDNF and intense synaptic activity will produce a long-lasting strengthening of the eCB system will be tested. 3) CB1 antagonists show promise for the treatment of opiate addiction. How prolonged CB1 receptor blockade affects the function of the eCB system is not known. The hypothesis that removal of a CB1 receptor antagonist following prolonged blockade of presynaptic CB1 receptors will result in enhanced endocannabinoid signaling will be tested. Overall, these studies will provide insight into whether the pre- and post-synaptic elements of the eCB system change in parallel during exposure to and withdrawal from opiates. Understanding the plasticity of the system as a whole will aid in developing agents to modulate eCB signaling during withdrawal and abstinence from opiates.